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Common Misunderstandings In The Selection Of Bolt Fasteners, See How Many Items You Have Won

Dec 30, 2020

Use coarse teeth instead of fine teeth


There are many important connecting parts on the machine, such as the drive shaft, and most of the bolts are fine thread. If there are missing parts during maintenance, some maintenance personnel will use coarse bolts instead, which should be avoided. Because the inner diameter of the fine pitch bolt is larger, the pitch and outer angle are smaller, the strength is high, the self-locking performance is good, and the ability to withstand impact, vibration and exchange loads is relatively strong. Once replaced with coarse-threaded bolts, it is prone to loosening or detachment, disassembly, and even mechanical accidents.


Pore mismatch


The bolts on the machine that bear the lateral load and shear force, such as the drive shaft bolts and flywheel bolts, are matched with the bolt holes as transitional fits. The assembly should be strong and reliable, and can withstand lateral forces. Someone does not pay attention to the inspection during the assembly process, and the installation continues when there is a large gap between the bolt and the bolt hole, which is prone to bolt loosening or cutting accidents.


Thicken nuts increase connection reliability


Some people mistakenly believe that thickening the nut can increase the number of working turns of the thread and improve the reliability of the coupling. But in fact, the thicker the nut, the more uneven the load distribution between the threads of each circle, and the easier the coupling is to loosen.


One female with multiple pads


After the installation is complete, sometimes the bolt is too long, so someone installs an additional spring washer on the bolt. In this case, the spring washer will break due to uneven force during the strengthening process, which will cause the bolt to be pre-tensioned. If the tightening force is reduced, eccentric load may be generated, which reduces the reliability of the bolt connection.


The tighter the better


Many workers have such a misunderstanding: that the bolts should be "tighter rather than loose", so they deliberately increase the tightening torque, resulting in bolt slip. In addition, some important bolts that need to be tightened with torque, some people use adjustable wrenches to save trouble. As a result, the torque is insufficient, which leads to the loosening of the bolts and even mechanical failure.


Too big washer will not get in the way


Sometimes there is a lack of washers of the right size, and some workers will replace them with washers with larger inner diameters. In this case, the contact area under the bolt head with the washers is small, which will reduce the bearing pressure or locking force of the washers. If the working environment is There are vibrations and shocks, and the bolts are easy to loosen.


Inappropriate locking


Important bolts should be locked with anti-loosening device after assembly. Here are four cases to explain. If a split pin is used for locking, it must not be locked with a thin split lock or half-piece split lock; if a spring washer is used for locking, it must not be used with a washer that has too small an opening error; if a lock is used for locking, the lock must not be locked in The edges and corners of the nut; if a double nut is used for locking, the thinner nut must not be installed outside.


False solidity


If the bolts, nuts or threads are rusted, or impurities such as scale, iron filings, etc., must be cleaned before assembly; burrs, sand and other impurities on the joint surface of the coupling must also be removed. Otherwise, when the bolts are tightened, they appear to be tightened on the surface, but in fact the couplings are not really tightened. Under such false solidity, the bolts will loosen quickly if vibration, load shock and temperature change occur.


The selection principle of fasteners


Determine the category


Standard fasteners are divided into ten categories, and the selection should be determined according to the use occasion and function of standard fasteners.


Bolts: Bolts are generally matched with nuts (usually plus a washer or two washers), and are used for connection and fastening occasions.


Nut: Use the nut and bolt to match.


Screw: The screw is usually used alone (sometimes with a washer), and generally serves as a tightening or tightening function.


Studs: studs are generally threaded at both ends (single-end studs are threaded at one end), usually one end of the rib is firmly screwed into the body of the component, and the other end is matched with the nut to play the role of connection and fastening. But to a large extent it also has a fixed distance function.


Washer: The washer is placed between the supporting surface of bolts, screws, nuts, etc. and the process supporting surface to prevent loosening and reduce the stress on the supporting surface.


Self-tapping screw: The screw hole of the workpiece matching with the self-tapping screw does not need to be tapped in advance, and the internal thread is formed while the self-tapping screw is screwed in.


Rivet: One end of the rivet has a head, and the rod has no thread. When in use, the rod is inserted into the hole of the connected piece, and then the end of the rod is riveted to play the role of connection or fastening.


Pin: The pin is inserted into the workpiece when in use, and usually serves as a connection or positioning.


Retaining ring: The retaining ring is usually on the shaft or in the hole, and plays the role of limiting the axial direction of the workpiece.


Wood screws: Wood screws are used to screw into wood for connection or fastening.


Determine standards and varieties


Selection principle of variety:


Considering the efficiency of processing and assembly, the variety of standard parts should be minimized in the same machinery or project.


From economic considerations, product standard parts should be preferred.


According to the expected use requirements of standard parts, the selection of varieties is determined in terms of type, mechanical properties, precision and thread.


Type:


Bolts: general purpose bolts: there are many varieties, including hexagonal head and square head, see national standards GB5780-GB5790, etc.


Bolts for reaming holes: When using, insert the bolts tightly into the reamed holes to prevent dislocation of the workpiece, see GB27, etc.


Stop bolt: There are square neck and vertebral points, see GB12-15 etc.


Special purpose bolts: such as bolts for T-slots, see GB37; joint bolts, see GB798; anchor bolts, see GB799, etc.


High-strength bolt connection pair for steel structure: generally used for friction-type connection of steel structures such as buildings, bridges, towers, pipeline supports and hoisting machinery, see GB3632, etc.


Nuts: general purpose nuts: mainly refers to hexagonal nuts, and there are also square nuts, with many varieties, see GB41, GB6170-6177, etc.


Slotted nut: mainly refers to the hexagonal slotted nut, that is, the groove is processed above the hexagonal nut. It is used in conjunction with screw bolts with holes and split pins to prevent the relative rotation of bolts and nuts, see GB6178-6181, etc.


Lock nut: refers to the nut with locking function, with nylon insert hexagonal lock nut, see GB889, GB6182, GB6183 and all-metal hexagonal lock nut, see GB6184-6187.


Special purpose nuts: such as disc nuts, see GB62; ring nuts, see GB63; cap nuts, see GB802, GB923; knurled nuts, see GB806, GB807 and embedded nuts, see GB809, etc.


Screw: Machine screw: Divided into many varieties due to different head and groove types. There are cylindrical head, pan head, countersunk head and half countersunk head type. The head shape is slotted (in-line material) and cross groove. See GB65, GB67-69 and GB818-820 etc.


Set screw: The set screw uses its tail end to prevent mutual displacement between the workpieces and transfer less torque. See GB71, GB73-75, GB77, GB78, etc.


Hexagon socket screws: Hexagon socket screws are suitable for occasions where the installation space is small or the head of the screw needs to be buried. , See GB70, GB6190, GB6191 and GB2672-2674 etc.


Special purpose screws: such as positioning screws, see GB72, GB828, and GB829; for screws that do not come off, see GB827-839, GB948, GB948 and ring screws, see GB825, etc.


Stud: unequal length double-ended stud: suitable for occasions where one end is screwed into the body of the component for connection or fastening. See GB897-900 etc.


Equal-length double-ended stud: suitable for matching the connecting end and the nut to play the role of connection or distance. See GB901, GB953, etc.


Washer: Flat washer: Used to overcome unevenness of the supporting surface of the workpiece and increase the stress area of the supporting surface. See GB848, GB95-97 and GB5287.


Spring (elastic) washers: See GB93, GB859, etc. for spring washers, and GB860, GB955, etc. for elastic washers.


Non-return washers: lock washers with internal teeth, see GB861; lock washers with external teeth, see GB862; single-ear lock washers, see GB854; double-ear lock washers, see GB855; round nut lock washers, see GB858, etc. .


Oblique washers: In order to adapt to the inclination of the workpiece supporting surface, oblique washers can be used. For square bevel washers with I-beam, see GB852; for channel steel, see GB853.


Self-tapping screw: Ordinary self-tapping screw: Thread conforms to GB5280 (thread for self-tapping screw), with a large pitch, suitable for use on thin steel plates or copper, aluminum, and plastic


Self-tapping locking screw: The thread conforms to the ordinary metric coarse thread, which is suitable for use in occasions requiring vibration resistance.


Rivets: hot-forged rivets: generally large in size, mostly used in locomotives, ships, boilers, etc., usually hot forging is required to shape the head. See GB863-866.


Cold heading forming rivets: The general diameter specification is ≤16mm, and the head is usually formed by cold heading. See GB867-870, GB109, etc.


Hollow and semi-tubular rivets: hollow rivets, see GB976; semi-tubular rivets, see GB873-875, etc.


Special purpose rivets: see GB1016 for headless rivets; see GB975 for tubular rivets; see GB827 for label rivets.


Blind rivet: It is a single-sided riveting rivet. It is a new type of rivet that has been widely used in recent years. It has the characteristics of speed, safety and convenience, and has the advantages of waterproof and anti-leakage.


Pins: cylindrical pins: cylindrical pins with or without thread, see GB119; cylindrical pins with internal threads, see GB120; cylindrical pins with external threads, see GB878; elastic cylindrical pins, see GB879; cylindrical pins with holes, see GB880, etc.


Tapered pin: with or without threaded cylindrical pin, see GB117; internal threaded cylindrical pin, see GB118; threaded tapered pin, see GB881; open-ended tapered pin, see GB877.


Split pin: Generally, it is matched with the screw bolt with hole and slotted nut to prevent the bolt and nut from loosening. See GB91.


Retaining ring: Retaining ring: Retaining ring with holes. See GB893; elastic retaining ring for shaft. See GB894 and GB896 for shaft opening retaining ring.


Wire retaining ring: wire retaining ring for holes, see GB895.1; wire retaining ring for shaft, see GB895.2 and wire locking ring, see GB921.


Locking retaining ring for shaft parts: retaining ring locked with taper pin, see GB883; retaining ring locked with screw, see GB884, GB885, etc.


Shaft end retaining ring: shaft end retaining ring fastened with screws, see GB891 and shaft end retaining ring fastened with bolts, see GB892.


Wood screws: Divided into many varieties due to different head and groove types. The head type has round head, countersunk head, half countersunk head and so on. The head groove shape is slotted (slotted slot) and cross slotted. See GB99-101, GB950-952, etc.